A solid polymer electrolyte fuel cell comprises a solid polymer electrolyte membrane that serves as an electrolyte and has a structure in which electrodes are separately bound to the both sides of the membrane.
It is necessary for a polymer solid electrolyte membrane to have low membrane resistance when used for fuel cells. For this reason, it is preferable that the membrane thickness be minimized. However, excessive reduction of membrane thickness is liable to result in pinhole formation during membrane forming, membrane breakage during electrode forming, or short circuits between electrodes, which has been problematic. In addition, whenever polymer solid electrolyte membranes are used for fuel cells, they are in a moist state. Therefore, moistening causes, for example swelling or deformation of polymer electrolyte membranes. This causes problems of durability in terms of pressure resistance, cross-leakage or the like during a differential pressure operation.
Hence, a thin reinforced membrane with a uniform thickness having uniform strength in both the longitudinal and lateral directions has been developed. For example, JP Patent Publication (Kokai) No. 2004-288495 A discloses a solid polymer fuel cell electrolyte membrane comprising a complex for which the tensile yield stress is 12 MPa or more in the longitudinal and lateral directions and the relative value of the tensile yield stress in the longitudinal direction to the tensile yield stress in the lateral direction (tensile yield stress in the longitudinal direction/tensile yield stress in the lateral direction) is 2.0 or less.
Meanwhile, JP Patent Publication (Kohyo) No. 2005-520002 A discloses a composite membrane that has extraordinarily improved hardness so as to reduce the occurrence of electric short circuits, thereby improving fuel cell performance and durability. In this case, an integrated composite diaphragm comprising stretched/expanded polytetrafluoroethylene, which has a morphological structure characterized by a fine structure of nodes with ultra-high extensibility (such nodes being bound to each other via fibrils), is used as an ion conductivity diaphragm having a high degree of hardness and dimensional stability and allowed to absorb ionomers.
In general, it has been attempted to form a composite of a porous body such as stretched polytetrafluoroethylene and an electrolyte material so as to reduce the occurrence of electric short circuits, thereby improving performance and durability. However, the porous body structure becomes complex. In order to improve membrane strength, proton conductivity (specifically, fuel cell performance) must be sacrificed, which is problematic.
Further, a polyelectrolyte material having high proton conductivity and excellent durability has been examined. However, when chemical resistance is imparted to such a membrane, the polymer structure becomes complex. This causes concerns of yield deterioration in the synthesis process and a sharp increase in material cost for synthesis of a novel material or the like. Furthermore, it cannot be said that sufficient polyelectrolyte material strength is achieved in such case. In addition to such problems, a membrane obtained by forming a complex of a polytetrafluoroethylene porous body and an electrolyte material has a membrane face with strength anisotropy. Accordingly, such membrane tends to become distorted in fuel cells, facilitating membrane deformation or destruction, which has been problematic.
The above problems have arisen due to lack of simultaneous achievement of improvement of electrolyte membrane strength and provision of chemical resistance. In addition, in order to improve strength based on conventional technology, it is necessary to increase the porous substrate thickness or change the fine porous substrate structure.
Hitherto, porosity has been imparted to polytetrafluoroethylene porous substrates by a stretching method. This often results in a difference between the degree of stretching in the machine direction (for sheet processing) (MD) and that in the transverse direction (TD; vertical to the MD direction). Therefore, it has been thought that it would be difficult to change the fine structure or reduce strength anisotropy in the MD and TD.